Ion chromatography is a form of commonly practiced ion analysis characterized by the use of two ion-exchange columns in series followed by a flow through conductivity detector. The first column separates the ions of an injected sample by elution of the sample through the column using as the eluent an electrolyte, i.e., usually dilute base or acid in deionized water. The second column, called the "suppressor" or "stripper", functions to suppress the electrical conductivity of the electrolyte in the eluent, but not that of the separated ions such that the latter may be determined by a conductivity cell. The technique is described in more detail in U.S. Pat. Nos. 3,897,213, 3,920,397 3,925,019 and 3,926,559. In addition, various ion-exchange packings useful in the separation column of ion chromatography are described in detail in U.S. Pat. Nos. 3,966,596, 4,101,460 and 4,119,580. A generally thorough description of ion chromatography is additionally given by Small et al., "Proceedings of an International Conference on the Theory and Practice of Ion Exchange", University of Cambridge, U.K., July, 1976; and also, Small et al., "Novel Ion Exchange Chromatographic Method Using Conductimetric Detection", Analytical Chemistry, Vol. 47, No. 11, September, 1975, pp. 1801 et. seq. The foregoing patents and literature publications are fully incorporated herein by reference.
One of the few drawbacks of ion chromatography is that the prior art resin bed type stripper column disadvantageously limits the number of samples which can be consecutively analyzed. As few as five samples, for example, may be run in certain types of analysis before regeneration of the stripper column is required. In addition, low capacity separating resins, e.g., 0.005-0.1 meq/g., and very dilute eluent, e.g., 0.001 to 0.05 molar electrolyte, are necessarily used for prior art ion chromatography, since higher capacity resins and/or higher concentration of electrolyte in the eluent would exhaust the stripper column too rapidly.
The prior art also describes certain difficulties with resin bed stripper columns which are considered to be caused by the variable length of nondepleted resin in the column as it is being used up. It has been observed, for example, that this factor can vary elution times of certain ions, but with less or no effect on other ions. Consequently, the resolution of ions achieved in the separator column can be lost or detrimentally affected in the stripper column. The foregoing literature references by Small et al. supra, and a more recent journal article "Complication in the Determination of Nitrite by Ion Chromatography", Analytical Chemistry, Vol. 51, p. 1571 (1979), discuss in more depth the above and various like complications which can result from technical problems with prior art stripper columns.
It is, accordingly, an objective of this invention to provide improved methods and apparatus for selectively suppressing the electrical conductivity of the electrolyte in the eluent but not the separated ions therein. It is a further objective hereof to provide improved ion chromatography methods and apparatus which allow for continuous analysis of consecutive samples without periodic regeneration or replacement of the stripper column. It is still a further objective of the invention to eliminate unsteady ion responses, i.e., particularly unsteady calibration and elution time variables which are a function of prior art stripper column exhaustion.